Both cellulose acetate and starch acetate have been known since the 19th century. However, a blend of these two common materials has not before been available due to processing problems.
Cellulose acetate of certain acetylation degrees is commonly used to manufacture cigarette tow, textile fibers, films, plastics and other materials. However, cellulose acetate as derived from wood pulp is expensive and requires the use of large amounts of acetone in processing resulting in high acetone recovery costs and possibly harmful acetone emissions. Starch acetate, depending on the degree of substitution (DS) of the free hydroxyl groups, may be used in a number of commercially important ways. Low DS acetates are important in food applications while highly derivatized starches with a DS of 2-3 are useful because of their solubility in organic solvents and ability to form films and fibers. Starch, to be used as a raw material for the preparation of starch acetate, is low cost and produces higher solids concentration in the acetylation process as compared with wood pulp acetylation.
A blend of cellulose acetate with starch acetate offers the possibility to combine a cheap raw material, starch, with the production of a fiber of the same or better quality and enhanced properties as compared with fibers made of cellulose acetate alone. The improved quality results from the molecular structure of starch as compared to wood pulp. Dry starch is generally a 25/75% mix of linear amylose and branched amylopectin polymer of .alpha.-D-glucopyranosyl units. Amylose has an average molecular weight of 17,000-225,000 with a degree of polymerization (DP) of about 100-1400. Amylopectin has an average molecular weight of 200,000--greater than 1,000,000. Dry wood pulp, the raw material used in the production of cellulose acetate, consists of 100% cellulose with an average molecular weight of 200,000--greater than 3,000,000 and a corresponding DP of 1200--greater than 20,000.
Despite the construction of the three polymers out of the same monomer, glucose, the branched structure of the amylopectin and the different conformations of the polymers is responsible for the enhanced properties of the blend versus either starch acetate or cellulose acetate individually. The lower molecular weight of amylose and amylopectin as compared with cellulose and the difference in molecular alignment, that is, in hydrogen bonding, results in less viscous acetone solutions of starch acetate which in turn allow an increased solids content in the spin dope. The spin dope is the cellulose acetate/starch acetate acetone solution. The increased solids content results in a spin dope of such a concentration to show liquid crystal behavior which generally results in higher strength fibers.
The advantages of a blend of starch acetate and cellulose acetate as noted above are evident, however, such blends in acetone have not previously been prepared because the acetone solution of the higher acetylated starch acetate (acetyl value (AV)=2.6-3) is immiscible with the standard cellulose acetate (AV=2.55) acetone solution.
It is known in the prior art to use blends of cellulose, acetate with other polymers to improve the properties of the blend. U.S. Pat. No. 2,059,425 discloses the production of textile materials and films using a blend of cellulose acetate with finely divided sugars such as, for example, glucose, sucrose or fructose, or their metallic compounds such as, for example, glucosates, fructosates, etc. of calcium, barium or strontium. The object of this patent is to deluster the produced plastic materials such as films or fibers by removal of the finely divided sugars by treating them with water or a solvent. The blend disclosed occurs only as an intermediate in the manufacturing process.
U.S. Pat. No. 3,272,638 discloses mixtures of cellulose acetate with sucrose and fatty acid esters or glucose fatty acid esters to make filaments such as filters tow. The fatty acids are acetic, propionic and isobutyric acid. One of the objects of this patent is to improve the spinning rate and curing process. The sucrose or esters are added to the acetone spin dope immediately before spinning.
U.S. Pat. No. 3,272,640 discloses a process for insolubilizing water-soluble materials such as cellulosics and starch derivatives corresponding to cellulosics by incorporating a "insolubilizing agent" with the water soluble material in a hydrophobic film firmer. The products obtained are claimed to be useful in the preparation of films and permanent fabric sizes with a high degree of water resistance. Both starch acetate and cellulose acetate are water insoluble materials.
U.S. Pat. No. 4,808,479 discloses a wrap yarn sizing composition that is comprised of a starch ether and/or ester having specific fluidity properties, however, the issue of a blend is not addressed in this patent.
Fibers spun out of a blend of starch acetate and cellulose acetate show significant improvements over fibers spun out of pure cellulose acetate. For instance, these fibers exhibit better dyeability, higher elongation due to the polymer structure, and improved moisture regain. Additionally, the use of a fiber spun out of a blend of starch acetate and cellulose acetate takes advantage of some physical properties imparted to the blend by the use of the starch acetate. For instance, there is a higher solids concentration in the spin dope which gives liquid crystalline behavior to the fiber resulting in higher tenacity.
The commercial significance of this invention lies not only in the desirability of an improved fiber but also in the environmental and economic aspects of this improved product. Cellulose acetate is typically produced through the acetylation and ripening of hardwood pulp in acetic acid with a sulphuric catalyst. Starch, a lower molecular weight raw material, can be similarly processed to make acetate. However, the cost of starch is approximately half that of wood pulp and the raw material is in much greater supply. Environmental aspects of this invention are related to lower acetic acid recovery costs and acetone recovery costs in the processing of starch acetate. The higher solids concentration in starch acetylation as compared to wood pulp acetylation results in lower acetic acid recovery costs, and accordingly, less emission of solvents in the extractions and distillation employed in recovery of the acetic acid. Additionally, higher solids concentration in the acetone spin dope reduces acetone recovery costs and acetone emissions.
The blend of the present invention is prepared by standard methods of preparing cellulose acetate.